4 wire flat to 3 wire on trailer

Our Tacoma has the 4 wire flat wiring harness and goes to a 3 wire on the trailer. For some reason we don't have the brake lights, but everything else works fine. Need to know where to start troubleshooting. Any ideas?

Test the plug on your truck first, have some one step on the brake and make sure the brake wire on your plug is hot. If its not then trace the wire from the plug into your truck harness and make sure its not damaged, just keep testing. If you visually cant see anything out of the ordinary then check your owners manual to see if there is a fuse for
trailer stop lamps. If there is then check to see if its blown or not. And if you do have power in your truck plug then start with the trailer, test from the plug to the tail lights to see if that wire is damaged. Thats where you can start let me know if you have anymore questions.